Update:

Just got back from the range.  The last round hold open issue on the 10.5" 5.56 is solved.  I had installed a Forward Controls Design bolt catch initially when I test fired.  I swapped for a USGI bolt stop and it worked flawlessly.

On the .300 Black build I did two things:  1) I slightly filed the very top edge of the bolt stop...just enough to round off the sharp edge at the top and 2) used the standard VLTOR A5 spring in place of the green Sprinco.  This seems to have solved the bolt lock issue. Unfortunately two new issues cropped up:

1) the empty cases are just dribbling out of the ejection port.  I may be getting 1 foot of ejection or so at 2-3:00 and occasionally a spent case would get hung up.  This was surprising since the cases were getting kicked out a good distance at about 4:00 last time.  

2) I put about 10-15 rounds downrange with no issue and made a quick adjustment to the optic (PA 3X micro prism) as I had to get the bullet strikes about an inch to the right.  I put another 3-5 rounds downrange and noticed that I wasn't hitting the target anymore.  I could see the dirt strikes but I wasn't even near the paper.  I checked the suppressor and didn't see any signs of a baffle strike. This is a Dead Air Sandman S on a KeyMo brake. I made sure the suppressor was tight on the mount (it was) and when I got home made sure the keymo brake hadn't started to work loose (it hadn't).

Either my micro prism crapped the bed and completely lost its zero, or my suppressor is now  throwing rounds about at least two feet to the left of aim now.

Ive got the suppressor recessed under the handguard a few inches so I couldn't shoot with it dismounted  I'm going to grab a spare optic and take the hand guards off so I can shoot with and without the suppressor to see what is going on.




Ive got two new builds that I test fired today that I need some help with:

Build 1:
.300 Black
8.5" Rainier barrel (pistol length gas)
non-adjustable gas block
Colt BCG
DeadAir Sandman-S
Silencerco ambi receiver (bolt hold open both sides)
Vltor A5 w/Sprinco Green Spring and A5H2 buffer
.300 Black specific PMAGs.  
Suppressor stayed mounted the whole time.  
Shooting factory 124 grain supers SIG and S&B) and 220 grain subs (Federal).  

Problem: On the supers the bolt would lock back with one or two rounds left in the magazine.  It wasn't consistent doing this.  Shooting subs I got last round lock back with no malfunctions.
The bolt face was past the  stop (not hung up on the mag or anything).
Ejection was at 3-4 o'clock. Tried 2 different mags.
Tore down the gun for cleaning and the top surface of the bolt hold open seemed slightly beat up with very slight peening where the bolt lugs would ride over
the top edge when the lever is depressed to run the bolt into battery.
I've tried to post a photo of this...hopefully it works.  
I was thinking maybe this was a BCG velocity problem on the supers but not sure. I probably should have swapped out the buffer for the H3 but I ran out of supers.

Build 2:

5.56 10.5" Criterion Core (carbine length gas)
Superlative adjustable gas block
Colt BCG, Aero standard lower
Vltor A5 with Sprinco Green (tried both A5H2 and A5H3 buffers)
Gen 3 PMAG
Shooting IMI 77 grain and 55 grain Federal
No suppressor

Problem: loading one round at at time in the mag I would consistently get last round hold open.  Loading more than one round in the mag (2, 6, 8, etc)
I would get consistent cycling with ejection at 4 o'clock but the bolt would not lock back on the last round.  
I tried both the H2 and H3 buffers.  I also confirmed the superlative was 18 clicks from the closed position and also tried opening the bleed a few clicks to see if that was the issue.  
Nothing seemed to help.  
Would lock back when loading one round in mag but anymore than 1 round it wouldn't hold open. I stupidly only brought one mag with me.

I appreciate any input on either problem.

Pics of the hold bolt stop from the .300 Black build:

Attached File

Attached File

Are the Colt BCG’s both FA carriers?

Yes both builds FA carriers

Do you have the ability to try the original A5 springs instead of the green sprinco’s?  I’m scratching my head with those issues, but I’ve also never used VLTOR’s A5 buffer system

I do have the original vltor springs. I’ll give that a try early next week when i can get back to the range. Does the wear on the bolt stop look odd to you? Ive seen wear on the ramp of the bolt stops  but ive never seen those shallow gouges from the lugs.

Yeah I’ve never seen that, was thinking that green sprinco is too strong and what could be causing it.  Someone smarter than me needs to chime in

You’re probably right.  Maybe 60 rounds total on the .300 balck and 40 rounds on the 5.56.  I was a bit rushed and ammo was limited.  I’ll get in it next week and see if I can run a hundred or si rounds through each and see what happens.

Both issues seem to involve the bolt catch.  Do both builds have that ambi bolt catch?  I've had issues in the past with those types of ambi bolt catches.  Swapping to milspec bolt catches always resolved the issues.  The added weight of those types of ambi bolt catches causes them to slow down and not rise up to catch the bolt in time after the last round.  Some may find a rifle that has a gas and buffer system setup just right for those to work, but I never had any luck.

@TITTB did you get this issue solved?  If so was it a bolt catch problem?

Sorry forgot to update this: the bolt catch issues with both builds is resolved.  The issue on the 5.56 build was the Forward Controls Design bolt catch. i swapped that out with a standard catch and no issues after that.  Witht the .300 black build i slightly rounded the very top edge of the catch with a file. Just enough to knock the edge off and that cleared up as well.  I wound up swappping the Rainier 8.5 1:8 for a Criterion Core 1:7 on the .300 black and added an adjustable gas block. And finally re. The update about loss of accuracy….that was 100% the Primary Arms 3x microprism.  i had a spare and put that on and it started grouping again.  I returned the microprism for a replacement and just got that a coulple of weeks ago. I was a bit surprised the 3x microprism failed. I hadn’t dropped it and had only out maybe 150-200 rounds total before it lost zero and started shooting about 2-3 feet to the left. Subsequent adjustments couldn’t re-zero it. PAvdidn’t indicate what was wrong with it in their emails…just that it was beyond repair and they were sending a replacement.